ECG measuring devices are primarily used for measuring and monitoring a patient's cardiac function. To that end the summation voltage of the electrical activity of the myocardial fibers is typically measured across at least two electrodes as what is referred to as an “ECG signal”.
There are further applications still, however. For example, ECG signals are also used in medical imaging for the purpose of generating trigger signals. During imaging, the ECG signal is used to acquire information about the cardiac cycle in order thereby to synchronize the imaging with the activity of the heart. In particular with imaging methods that require a relatively long recording time, high-quality images of the heart or also images of regions that are moved by the heartbeat can be recorded in this way.
ECG measuring devices are also used for recording ECG signals during an examination of a patient by means of a magnetic resonance scanner. In this case, however, operation in the magnetic resonance scanner places particular requirements on the ECG measuring device due to the strong gradient fields and radio-frequency fields used for imaging in the scanner, in order to prevent mutual interference between magnetic resonance scanner and ECG measuring device. ECG measuring devices which are magnetic resonance compatible in the sense referred to above are available on the market.
However, a continuing major problem for reliable ECG signal measurement are magnetic fields that change over time, as used in the magnetic resonance scanner as magnetic gradient fields for position encoding. According to the law of induction, temporally fluctuating magnetic fields of this type generate interference voltages which are coupled into the ECG signal recorded by the ECG electrodes as noise. A further source of induced interferences of said kind are movements of the ECG measuring device in the static magnetic field of a magnetic resonance scanner. Such magnetically generated interference signals are superimposed on the ECG signal generated by the heart and distort said signal.
These interference sources are extremely undesirable. Synchronizing a recording of a magnetic resonance image with the heartbeat requires reliable detection of the R wave of the ECG signal. Due to their often similar shape, for example, the interference signals can erroneously be interpreted as an R wave and consequently can incorrectly initiate a triggering of a recording of a magnetic resonance image. On the other hand it can also happen that a “real” R wave is not recognized as such because of the superimposed interference signals. This frequently leads to a significant deterioration in image quality.
In the prior art, attempts have been made to avoid incorrect triggerings of said kind by deductions based on an analysis of the amplitudes and dynamics of the ECG signals.
Further sources of errors in ECG signals can be due to the manner in which an ECG measuring device used is installed and set up. For example, a loose connection or a break in an electrode cable may be present.